Our section on cytoskeletal protein regulation has studied the phosphorylation of neuronal cytoskeletal proteins and its regulation in mammalian and squid giant neurons. Initially we focused on kinases responsible for the extensive phosphorylation of the numerous KSP repeats in tail domains of neurofilament proteins and found that among several kinases, the proline directed kinases such as cdk5 and Erk1/2 were primarily involved. In addition we demonstrated in mammalian and squid neurons that phosphorylation was topographically regulated; i.e., tail domain phosphorylation was restricted to the axonal compartment although kinases, regulators and substrates were also present and active in the perikaryal compartment. Meanwhile studies in our lab and in other laboratories implicated cdk5 as a tightly regulated multifunctional kinase in the nervous system, important in neuronal migration during development, and essential for survival. Weve shown that a cdk5 KO is lethal with major defects in the nervous system. When cdk5 is deregulated in neuronal stress and/or neural degeneration it can lead to abnormal perikaryal accumulations of phosphorylated cytoskeletal proteins such as tau and neurofilaments, leading to cell death. These two problems, the topographic regulation of cytoskeletal protein phosphorylation in neurons and the role of cdk5 in neuronal function, development and survival emerged as the principal programs for this laboratory over the past decade. Cdk5 crosstalk regulation and neuronal survival In previous studies we demonstrated that (1) cdk5 upregulates the PI3K/Akt neuronal survival pathway by phosphorylation of a key neuregulin receptor which, in turn induces Akt phosphorylation and (2) cdk5 phosphorylation of JNK 2 inhibits the JNK apoptotic pathway and protects neurons from experimentally induced stress. These observations suggest that in neurons, where cdk5 is particularly active, cdk5 plays an important role in modulating the activity of signal transduction pathways. To further examine this role, we recently studied the cross talk between cdk5 activity and the MAPK pathway, particularly in stress situations where sustained activation of Erk1/2 activity leads to cell death. We demonstrated that cdk5 phosphorylates and inhibits MEK1, that inhibition of cdk5 in cortical neurons by roscovitine or DNcdk5 leads to sustained activation of Erk1/2, abnormal phosphorylation of tau and NFs, leading to cell death. These results suggest that cdk5 may play a surveillance role in modulating the activities of several signal transduction pathways to promote neuronal survival. Based on studies of AD brains and mouse model systems it has been hypothesized that deregulation of cdk5 activity is responsible for some of the pathology of neurodegeneration, the appearance of hyperphosphorylated tau PHF in neuronal perikarya; i.e., topographic regulation of cytoskeletal protein phosphorylation has been compromised. Deregulation of cdk5 activity seems to arise as a result of the stress-induced cleavage of the p35 activator to a 10kd fragment and a p25 hyperactivating regulator. The cdk5/p25 complex is considerably more stable and active than the normal cdk5/p35 complex and promotes abnormal tau and NF hyperphosphorylation. We have already shown that a 125 amino acid fragment from p35 (CIP) inhibits the cdk5/p25 hyper- phosphorylation of cytoskeletal proteins in vitro and in vivo without affecting the normal activity of cdk5/p35. This suggests that cdk5/p25 may be a target for therapeutic drugs for neurodegenerative disorders.